1. The Field of the Invention
The present invention relates to hydraulically settable compositions and methods used to extrude a wide variety of articles therefrom. More particularly, the invention relates to hydraulically settable mixtures which have rheological properties that render them highly extrudable under pressure and then form-stable immediately after they have been extruded, even while in the green or unhardened state. The compositions and methods allow for the extrusion of articles having thin walls, complicated shapes, or highly critical tolerances.
2. The Relevant Technology
Hydraulically settable materials such as those that contain a hydraulically settable binder like hydraulic cement or gypsum (hereinafter "hydraulically settable," "hydraulic," or "cementitious" compositions, materials, or mixtures) have been used for thousands of years to create useful, generally large, bulky structures that are durable, strong, and relatively inexpensive. Hydraulic cement is a hydraulically settable binder derived from clay and limestone, while gypsum is a naturally occurring mineral. Both are essentially nondepletable.
Hydraulically settable materials are generally formed by mixing a hydraulically settable binder with water and usually some type of aggregate to form a hydraulically settable mixture, which hardens into, e.g., concrete. Typically, a freshly mixed hydraulically settable mixture is fairly nonviscous, semi-fluid slurry, capable of being mixed and formed by hand. Because of its fluid-like nature, a hydraulically settable mixture is generally shaped by being poured into a mold, worked to eliminate large air pockets, and allowed to harden. If the surface of the concrete structure is to be exposed, such as on a concrete sidewalk, additional efforts are usually made to finish the surface to make it more functional and to give it the desired surface characteristics.
Due to the high level of fluidity required for typical hydraulically settable mixtures to have adequate workability, the uses of concrete and other hydraulically settable materials have been limited to mainly simple shapes which are generally large, heavy, and bulky, and which require mechanical forces to retain their shape for an extended period of time until sufficient hardening of the material has occurred. Another aspect of traditional hydraulically settable mixtures or slurries is that they have little or no form-stability and are usually molded into the final form by pouring the mixture into a space having externally supported boundaries or walls. The problem of low form-stability is exacerbated by the lengthy curing and hardening times of most concretes. It may take days for most cementitious mixtures to have enough strength to be demolded without being harmed, and weeks of wet curing to avoid defects in the structural matrix.
The uses of hydraulically settable materials have also been limited by the strength properties of concrete, namely, the high ratio of compressive strength to tensile strength, which is usually about 10:1. Fortunately, the strength limitations of concrete can usually be overcome by simply molding it into massive structures of enormous size. This is possible because of the extremely low cost of most concretes. The tensile strength of such massive structures has also been improved by extensive use of metal reinforcing bars, or "rebar". However, the use of such hydraulically settable materials to make smaller, thinner-walled articles has not been possible. The inability to use conventional cementitious materials in the manufacture of small, thin-walled objects is further complicated by the low strength per unit weight of the composition. In light of these factors, concrete is ideally suited for manufacturing large, bulky, and massive objects such as curbs, walkways, highways, bridges, buildings, aqueducts, sewer pipes, etc.
The long time period which must pass before typical hardened cementitious products can be demolded greatly impedes the ability to mass produce cementitious objects and increases the cost of the final product. The lengthy period before demolding also makes necessary the initial investment of substantial amounts of money in order to purchase the large number of molds required to mass produce a particular shape or article.
One way to avoid the added difficulties of having to demold the large, molded concrete forms would be to create a cementitious mixture with sufficient viscosity and cohesive strength so that it could be extruded into the desired shape without a mold and then maintain that shape (i.e., have form-stability). Extrusion of such a material would allow for the continuous production of hydraulically settable objects without the use of molds. The term "extrusion" has often been misleadingly used in many patents and articles to denote processes for laying out a continuous layer of concrete in a mold, which is then often compacted. Some have attempted to "extrude" cementitious mixtures, although the objects extruded therefrom have mainly consisted of slab-like objects, relatively thick-walled pipes, and other structures of simple shape.
The extrusion processes that do not require a mold, mandrel, or other supporting structure often involve the vertical (or downward) extrusion of materials to eliminate gravitational deformation of the structure. One problem with this is the discontinuous nature of the downward extrusion process, since the extruded structures will eventually fail under their own weight. Another drawback has been that to obtain structures that would maintain their shape and not slump when extruded it has been necessary to use highly viscous cementitious materials that require correspondingly high extrusion pressures and energies (such as vibration). In most cases, true form-stability (in which the object will not slump without supporting mandrels, or in the vertical position) is not achieved for a significant period of time, usually minutes or even hours. The use of fibers to improve the material properties of the final cured materials has further compounded the difficulties of the prior "extrusion" processes.
An example of an "extrudable" cementitious mixture can be found in U.S. Pat. No. 3,857,715 to Humphrey. Humphrey discloses a cementitious mixture having a low water content, a carbohydrate additive (such as molasses), and a high pH regulator, and which is said to be "self-supporting before being cured" (col. 3, lines 19-20), at least with respect to the objects which were reportedly formed by extrusion, namely "concrete block and pipe." (col. 1, line 7).
Nevertheless, Humphrey only partially solves the problems typically associated with the molding of cementitious materials, which have prevented their use in manufacturing a variety of useful objects which must presently be made from materials having superior properties of strength or moldability (such as paper, paperboard, plastic, polystyrene, or metal). The cementitious mixtures disclosed in Humphrey are only useful in the manufacture of relatively large, bulky objects of extremely simple shape, such as concrete slabs or relatively thick-walled pipes (measured by the ratio of wall thickness to cavity cross-section). There is no indication that the compositions in Humphrey can be molded or extruded into more complicated structures or objects having relatively thin walls or highly critical tolerances.
In addition, the cementitious mixtures disclosed in Humphrey obtain their higher level of plastic-like behavior and form-stability through the use of sugar-based additives, together with a strong base, such as NaOH, and set accelerators, such as CaCl.sub.2. The use of these additives in these combinations diminishes the ultimate strength of the final cured cementitious product. Furthermore, in spite of the use of accelerators, the use of sugars to thicken the cementitious composition in Humphrey greatly retards the hydraulic reaction of the cement binder to the point that the extruded materials will sag or slump over time because of the inability of the mixtures to hydrate, and thereby solidify, in a timely manner.
U.S. Pat. No. 5,047,086 to Hayakawa et al. discloses a cementitious composition suitable for "extrusion molding" into the shape of boards having nailing and sawing characteristics similar to that of wood. In order to provide a composition with adequate extrudability, a cellulose-based additive is blended with the cementitious mixture in an amount of 0.2% to 1% by weight, along with cellulosic fibers. However, Hayakawa et al. does not disclose or teach that the cementitious mixture described therein would be useful in creating "extruded" articles other than flat boards. Nor does Hayakawa et al. anywhere indicate that the cementitious materials disclosed therein would have adequate strength, toughness, or other performance criteria that would allow it to be extruded into anything other than relatively thick boards having a simple, rectangular shape.
U.S. Pat. No. 4,588,443 to Bache discloses a densified cementitious product known in the art as "DSP", which has superior compressive strength properties and which is ideal for a number of uses requiring high compressive strength concrete. Within Bache are drawings showing a cementitious mixture being "extruded" onto a form, or extruded to form a continuous strip which is then spiral wound onto a mandrel to form a pipe. However, nowhere does Bache teach that the cementitious materials disclosed therein might be extruded into more complicated structures, including multicellular or thin-walled objects, which will then maintain their shape without external support. Moreover, Bache fails to teach how to obtain a hydraulically settable mixture that is form-stable in the green state.
A major reason why cementitious materials, including those disclosed in Humphrey, Hayakawa et al., and Bache, are not capable of being continuously extruded into complicated or thin-walled shapes is the tradeoff between workability and form-stability. The level of workability and flowability is substantially constant in all known cementitious compositions throughout the three phases of extrusion: (1) placing the mixture into the extruder opening, (2) applying a pressure to the mixture to cause it to flow through the extruder, and (3) allowing the mixture to pass through a die orifice of the extruder. Although a stiff product might be greatly desired after step (3), it cannot be so stiff as to prevent the flowability of the mixture during step (2). Vibration of the mixture during steps (2) and (3) is often necessary in order for the mixture to flow or be "extruded."
In order for a hydraulically settable mixture to be capable of being extruded using ordinary auger extruders used to extrude clay or plastic, the stiffness of the hydraulically settable mixture may not exceed the level which will prevent it from flowing through the extruder (or which would only yield a dry, noncohesive extrudate). However, the maximum allowable stiffness which will still allow the hydraulically settable mixture to flow through the extruder has heretofore yielded an extruded product having inadequate form-stability for all but the simplest extruded articles, such as the cement slabs or thick-walled pipes disclosed in Humphrey or Hayakawa et al. In most cases, the "extruded" cementitious mixture must be placed into a supporting form or onto a mandrel in order to maintain the "extruded" material in the desired shape, such as in Bache.
As the article being extruded becomes thinner-walled (by using a die head with smaller openings), greater amounts of pressure and workability are required to overcome the increased resistance to flow of the material. However, greater cohesive strength and form-stability are also required in order for an extruded material having thinner walls to maintain its shape without external support. For this reason, cementitious mixtures have not been used to extrude anything but the simplest of objects because they have not been able to simultaneously have adequate extrudability while yielding a product with adequate form-stability.
Because of the tradeoff between workability (and extrudability) and form-stability, cementitious materials which are "extruded" horizontally according to conventional methods generally must have a wall thickness of about 25% of the cavity cross-section in the case of hollow objects (e.g., pipe), although vertically extruded objects can have a wall thickness to cavity cross-section up to about 1:16. Furthermore, conventional cementitious mixtures typically cannot be continuously extruded into objects that are very long. For example, the length to width ratio of most extruded objects has typically peaked out at about 5:1.
Even when fully cured, typical cementitious materials, even those disclosed in Humphrey, Hayakawa et al., and Bache, have relatively low tensile and flexural strengths compared to other materials such as paper, metal, or plastic. On the other hand, typical cementitious mixtures, particularly those such as DSP, have a correspondingly high level of compressive strength to the point that it is unnecessary and, hence, inefficient. The relatively low tensile and flexural strengths, along with the problems with molding or forming cementitious mixtures, limit the use of cementitious mixtures to mainly large, bulky, heavyweight objects. Consequently, it would be a tremendous advancement in the art if a wider variety of articles having complex shapes or highly critical tolerances could be manufactured from cementitious mixtures, particularly in light of the extremely low cost of cementitious materials compared to most other materials.
In fact, it is completely contrary to human experience to imagine the manufacture from hydraulically settable materials of small, thin-walled, relatively lightweight articles which are presently manufactured from lighter weight, yet higher strength, materials such as paper, paperboard, plastic or other polymers, and aluminum or other metals. It would be an even greater achievement if such hydraulically settable objects could be mass-produced in an economical and cost-effective manner.
Due to a growing awareness of the environmental harm caused by the massive use of paper, paperboard, plastic, metal, and wood to manufacture the large quantities of mainly disposable items, there has been an acute need to find environmentally sound substitutes for such materials, such as hydraulically settable materials. In spite of such pressures and long-felt need, the technology simply has not previously existed for the economic and feasible production of hydraulically settable materials which could be substituted for paper, paperboard, plastic, polystyrene, metal, or wood in making a huge variety of articles. From an ecological standpoint, such a substitution of materials would greatly reduce the amount of essentially nondegradable, environmentally harmful refuse which continues to build up within the nation's ever dwindling landfills.
Hydraulically settable materials are environmentally sound because they essentially include aggregates consisting of natural geologic materials, such as sand and clay, which are bound together by the reaction products of a hydraulically settable binder and water, which is also essentially "rocklike" from a structural, and especially chemical, viewpoint. Hydraulically settable materials have essentially the same chemical and structural composition as the earth into which such materials might eventually be disposed.
In addition, paper, plastic, metal, and wood are far more expensive than typical hydraulically settable (including cementitious) materials. Because no rational business would ignore the economic benefit which would necessarily accrue from the substitution of radically cheaper hydraulically settable materials for paper, paperboard, plastic, metal, or wood materials, the failure to do so can only be explained by a marked absence of available technology to make the substitution.
Based on the foregoing, it would be an advancement in the art to provide compositions and methods that would allow the extrusion of hydraulically settable materials into articles and shapes which have heretofore been impossible because of the inherent strength and moldability limitations of presently known hydraulically settable compositions.
It would yet be a tremendous advancement in the art to provide compositions and methods which result in the ability to extrude hydraulically settable products that have high strength in the green state. Such composition and methods would be particularly useful if the extruded products were immediately self-supporting without external support.
It would further be an advancement in the art to provide hydraulically settable compositions which were highly plastic or moldable and which would readily maintain whatever shape into which they were extruded.
It would be an even greater advancement if such extruded hydraulically settable materials could be handled and transported using conventional handling means.
Still, it would be an advancement in the art to provide compositions and methods which would yield a variety of thin-walled hydraulically settable articles. In addition, it would be a tremendous advancement in the art to extrude hydraulically settable articles having highly critical tolerances or dimensional preciseness.
It would yet be an advancement in the art to provide compositions and methods for the extrusion of hydraulically settable articles having an increased tensile strength to compressive strength ratio compared to conventional hydraulically settable materials.
It would be a tremendous advancement in the art to provide compositions and methods which could be used to extrude hydraulically settable articles that could take the place of articles presently manufactured from other materials, such as paper, paperboard, plastic, clay, metal, or wood.
In addition, it would be a significant improvement in the art if such compositions and methods yielded hydraulically settable articles which were environmentally benign which essentially consisted of the components found naturally within the earth.
It would be an advancement in the art to provide hydraulically settable compositions that had the rheology and plastic-like behavior of clay such that such compositions could be extruded using a clay extruder.
From a practical point of view, it would be a significant improvement if such compositions and methods made possible the continuous manufacture of hydraulically settable articles at a cost and at production rates (i.e., high quantity) that are comparable or superior to the cost of manufacturing such articles from paper, paperboard, plastic, clay, metal, or wood.
Such compositions and methods are disclosed and claimed herein.